(1) Field of the Invention
The present invention relates to a numerical control machine (NC machine), and more particularly, to a numerically controlled lathe having a plurality of tool rests or tool posts.
(2) Description of the Related Art
Japanese Unexamined Patent Publication No. 56-69003 filed by this assignee discloses a numerically controlled lathe (NC lathe) which includes a saddle slidably disposed in parallel to the longitudinal axis of the spindle of the headstock and having a lateral guide path perpendicularly across the longitudinal axis of the headstock spindle, and a lateral mount slidably mounted on the lateral guide path of the saddle and having two turret heads disposed on both sides of the spindle axis. In this NC lathe, while a working operation is carried out by using one of the tools held by one of the two turret heads, it is possible to index a selected one of the tools held by the other turret head, whereby the indexing time as required in a conventional NC lathe can be virtually eliminated so that an idling time of the NC machine can be shortened.
However, with the arrangement as mentioned above, it is impossible to immediately start a successive working operation by use of the indexed tool just after the preceding working operation is finished, because an access time is needed for moving the indexed tool to an initial working position. Therefore, in this connection, there still remains room for improvement of the prior NC lathe, to enhance the productivity thereof.
In addition, the construction of the prior NC lathe does not make it possible to carry out a working operation by simultaneously using two or more tools for the purpose of increasing the productivity and/or workability thereof.
On the other hand, a Swiss-type screw machine, which is very well known in this field, includes a plurality of tools which are perpendicularly and radially disposed around the longitudinal axis of the headstock spindle so that the respective tools are individually accessible to a workpiece along their axes, whereby not only can a selected one of the tools be on standby, so that a successive working operation can start immediately a preceding working operation is finished, but also a working operation can be achieved by a simultaneous use of two or more tools. However, it is not expedient to incorporate a numerical control unit into the Swiss-type screw machine because the numerical control unit per se, as well as a program to be used therein, are very complicated, as many tools must be individually controlled thereby, and because the controlling must be so performed that the tools do not interfere with each other. Although the Swiss-type screw machine having many control axes on the tools may be numerically controlled, such a numerical control machine would become bulky and be very expensive.